1. Field of the Invention
The invention relates to a sealable, opaque, biaxially orientated multilayer polypropylene film comprising a core layer, an intermediate layer located on one side or both sides thereof and a top layer located on one or both sides of the intermediate layer or the core layer. The invention furthermore relates to a process for the production of the multilayer film and its use.
2. Description of Related Art
Opaque films are generally known. For example, a five-layer sealable film which has a vacuole-containing base layer produced from propylene homopolymer and calcium carbonate, and an intermediate layer of propylene homopolymer and hydrocarbon resin, and sealable top layers comprising propylene/ethylene copolymer, is described in EP-A-180, 087. Because of the thick intermediate layer of polypropylene and hydrocarbon resin, the film has better mechanical properties and a higher gloss than conventional films. In addition, the film is suitable for corona treatment because of the impressed-on sealing layers of propylene/ethylene copolymer. The film is in need of improvement in its gloss and whiteness or opacity. The non-optimum gloss results from the top layer having a customary thickness of 0.5 to 2 .mu.m, and the low opacity values are due to the thick transparent intermediate layers. Only a low proportion of the thickness remains for the vacuole-containing core layer.
A multilayer opaque film in which one top layer consists of a readily printable polymer and which has an intermediate layer of propylene homopolymer and a vacuole-containing core layer is known from EP-A312,226. The film is said to be distinguished by a good gloss when viewed from the homopolymer side. Because of the stated layer thickness ratios, as in the case of EP-A-180,087, the surface gloss and the opacity or whiteness are in need of improvement.
A film of high whiteness and high opacity which can readily be printed and is said to be glossy is described in EP-A-408,971. Here also, the gloss is in need of improvement because of the high thickness of the top layer. On the other hand, the film may have a cloudy structure, which probably originates from the small particle size of the calcium carbonate used. The particle size of 1.0 .mu.m, which is stated as preferred, results in a poor dispersibility in the polymer. In addition, the increased fine content of the grains leads to a reduced opacity, since this means that no vacuoles are formed.
Readily printable opaque films are described in EP-A-234,758 and in EP-A-225,685. However, because of the nature of the surface, the films are distinguished by a very matte appearance.